1. Field of the Invention
This invention relates to a disc brake operated by hydraulic pressure and more particularly to an automatic gap adjustment device for the disc brake which is capable of automatically adjusting a clearance or gap formed between a rotating disc rotor and pads arranged to be pushed against the disc rotor.
2. Description of the Prior Art
The automatic gap adjustment device for a disc brake has been disclosed, for example, in the patent specification of British Pat. No. 1,540,084. In the case of this patent specification, a reversibly screw coupled combination of a nut and a push rod is inserted into each of a pair of pistons. An end face of the nut is pushed against one of the pistons by means of a spring to restrain the nut from turning round. Meanwhile, the push rod is arranged to turn round and screw outward to move its head part away from a member limiting the rotation of the push rod when hydraulic pressure acts between the two pistons. A clearance or gap between a disc rotor and pads is thus arranged to be automatically adjusted to prevent it from exceeding a necessary range.
However, the above-stated automatic gap adjustment device has a drawback in that a working hydraulic liquid supplied to a part between the pair of pistons tends to leak to an atmospheric pressure space on the side of the head part of the push rod. More specifically, the device disclosed by British Pat. No. 1,540,084 has the above-stated nut inserted in a blind hole of one of the cup-shaped pistons. Meanwhile, the push rod which is in screwed engagement with the nut is inserted into the other piston which has a through hole in the head part thereof. The leak of the working hydraulic liquid to the head side of the push rod is arranged to be prevented by mounting a seal ring on the push rod. However, as mentioned above, the push rod is reversibly screw coupled with the nut and is arranged to turn round in the screwing-out direction when gap adjustment is made when the hydraulic pressure acts between the two pistons. This causes the sealing ring which is mounted on the push rod to turn round together with the push rod. Therefore, the working hydraulic liquid tends to leak.
Further, to prevent excessive adjustment, the above-stated device is arranged to restrain the push rod from rotating by a rotation limiting member which is arranged to come into frictional contact with the head part of the push rod. However, the head part of the push rod comes into contact with the rotation limiting member when cylinders, etc. are elastically deformed by high pressure acting between the two pistons. Therefore, the degree of hydraulic pressure required for preventing the excessive adjustment greatly varies with the coefficient of friction between the head part of the push rod and the rotation limiting member. The arrangement of the device thus tends to result in excessive adjustment.